Question.

A 56yr old man with previous cardiac disease presents with palpitations and a broad complex tachycardia on his ECG. Which one of the following electrocardiographic features would indicate that the rhythm is ventricular tachycardia as opposed to a supraventricular rhythm with aberrant conduction?

a. absent concordance throughout the chest leads
b. J waves seen at the end of each QRS
c. absence of visible p waves
d. fusion beats and capture beats
e. rate greater than 160 beats/minute

 

Answer.
The correct answer is d. The presence of fusion beats and capture beats indicates that a broad complex tachycardia is ventricular in origin (ventricular tachycardia).
 
Discussion.
Broad complex tachycardia is frequently seen in the ED and is caused by either ventricular tachycardia (VT) or a supraventricular rhythm with new or pre-existing aberrant conduction (bundle branch block).   While as a rule, all broad complex tachycardia should be treated as ventricular tachycardia it can desirable to diagnose the underlying rhythm more accurately to give more focused management.

Electrocardiographic features indicating VT rather than SVT with aberrancy include:

– concordance of QRS polarity across the chest leads (figure. 1).
– AV-dissociation, visible P waves at sinus rate but with no relationship to the broad QRS complexes
– capture beats, an impulse from the SA node transiently captures the ventricle and intermittent narrow complex QRS complexes are seen
– fusion beats, a ventricular beat simultaneously triggered by both a sinus impulse and the ventricular tachycardia such that an occasional odd looking hybrid QRS complex is seen
– very broad complexes (>160ms)

Atrio-ventricular dissociation occurs in around half the episodes of VT where, despite the independent ventricular rhythm, the sinus node is still firing and sinus impulses are still reaching the ventricle via an intact atrioventricular conduction system.  P waves, capture beats and fusion beats all provide evidence of A-V dissociation in a broad complex tachycardia and their presence indicate the underlying rhythm is VT.  Of course, the likelihood of a broad complex tachycardia being VT is also increased in those with structural heart disease, ischaemic heart disease and/or heart failure.

Figure.1 Ventricular tachycardia (VT) with positive concordance across the chest leads